A rotating recording disk, such as an optical disk, can be formatted in concentric physical tracks (each corresponding to a single disk revolution) or in a single spiral around a central spindle hole/mounting hub. The spiral is divided into a series of logical tracks (not necessarily corresponding to a single revolution); each logical track (hereinafter referred to simply as a "track") is subdivided into sectors onto which blocks of data are recorded. Each sector has a header field followed by a data field, with an intra-sector gap between the two. An inter-sector gap separates the sector from a following sector. The header is generally embossed into the surface of the disk during the manufacturing process (although the disk can be soft formatted instead) and includes a sector mark, a series of identification (ID) fields with track and sector information, and other fields to aid a read/write controller in synchronizing to the sector. The data field includes user data, associated error detection and recovery information, and synchronization information. Under one proposed standard for read only, write once and rewritable optical disk cartridges (in which an optical disk is encased in a protective housing), the header is 52 bytes long (including a five-byte, five feature sector mark pattern) and the data field is 1274 or 665 bytes long (based on 17 or 31 sectors per track, respectively, and able to record user data of 1024 or 512 bytes, respectively).
During a read or write operation, a laser beam emitted from a read/write head must be directed onto the sector to be accessed. As is known, the head is moved radially inward or outward on a carriage under servo control for the laser beam to "land" on a sector near, but ahead of, the target sector. "Near" is between about five tracks before and three sectors before the target sector. The drive controller switches to a track following mode and, using asynchronous detection, attempts to detect the first sector mark encountered by detecting laser light reflected off of a surface of the disk. A sector mark is considered detectable if at least three of the five features in the sector mark pattern are recognized. The light reflections are converted into electrical signals representative of the data recorded on the disk. If at least three features cannot be satisfactorily detected, synchronization with the disk format is not achieved and the head scans for another sector mark. Once a sector mark is satisfactorily detected, the drive controller activates a phase locked loop to lock onto the incoming identification data stream of the rest of the header and to synchronize the drive controller to the data stream. Current disk formats employ a pulse position modulation (PPM) format although other formats have been proposed. The ID data is decoded into track and sector information, enabling the drive controller to determine the exact position on the disk of the laser beam.
If the sector on which the laser beam lands is not the target sector, a conventional controller reads subsequent sector marks and headers as the laser beam spirals toward the target sector. After detection of a first sector mark and ID, detection of subsequent ID's is commonly made more robust by disabling sector mark detection except during a small timing window (such as about .+-.0.5%) about the nominal location of subsequent sector marks to prevent false sector mark detection. Another commonly used technique forces a false (or "pseudo") sector mark detection event at the end of the timing window if no sector mark has been detected within the window. Such a technique allows the reading of an ID which has a defective or destroyed sector mark. By thus windowing in time from one sector mark to the next, a more robust ID reading system is obtained in which false sector marks are ignored and bad sector marks do not inhibit reading of the ID. In this manner, the controller counts down ID's until the target sector is reached. When the target sector is reached and properly identified, the user data is read from, erased from or written to the data field.
When the disk spins at a constant angular velocity (CAV), data recording and reading is highly stable and access time to a target sector is relatively fast. However, if a constant recording data rate is used, data recorded on tracks near the outer diameter of the disk will be at a lower linear density than data recorded near the inner diameter, although the amount of data recorded in a given angular rotation (angular recording density) will be the same. To increase the recording capacity of the disk, the linear density of the data should remain substantially constant by increasing the angular recording density as the radial distance from the spindle hole increases. Recording at a constant linear velocity (CLV) by decreasing the angular velocity (rotational speed) of the disk with increasing radial distance and maintaining a constant recording or data transfer rate can achieve increased angular recording density. But, random access time to a target sector is increased due to the time required to change the rotational speed of the disk.
A method which provides the advantages of both CAV and CLV, while reducing the disadvantages of each, is to record at a modified constant angular velocity (MCAV) by increasing the data transfer rate of the drive controller with increasing radial distance while holding the angular velocity constant. The transfer rate is a function of the frequency of the controller's clock and can be increased continually or can be increased incrementally by grouping tracks into bands and incrementing the clock frequency from band to band.
As an example of banded media, optical disks formatted according to the previously mentioned standard have 37,600 tracks in a user zone (located between inner and outer manufacturer and control zones near the inner and outer diameters of the disk) divided into sixteen annular bands of between 1600 and 3100 tracks each, the number of tracks per band increasing with increasing radial distance from the center spindle hole. Relative to a base frequency "f", the clock frequency is "16f" at band zero (the innermost band), while at band fifteen (the outermost band), the clock frequency is "31f". Thus, the angular recording density will increase band by band with increasing radial distance but the linear density at the inner diameter of each band is constant.
Spiral, banded recording is generally preferred for high performance optical applications. If during a seek operation the laser beam lands in the same band in which the target sector is located, sector marks, ID information and user data can be detected and read without a clock frequency change. However, if the laser beam lands in one band (band N-1) and the target sector is in the next band (band N), the clock frequency must be set to f.sub.N-1 for proper sector mark detection and to read the ID's in band N-1 and then changed to frequency f.sub.N at the boundary between bands N-1 and N for proper sector mark detection and to read the ID's and user data in band N. Because of code overhead, clock settling time and other delays, which can be approximately 50 microseconds, a certain delay is incurred after the clock frequency has changed. This clock switching delay prevents the use of windowing to the next sector mark and reduces the robustness afforded by the windowing technique. For example, if the first sector of the new band has a bad sector mark, it may be unrecoverable with no means to synchronize the controller to read the ID. Moreover, a sector near the inner diameter of a 512 byte/sector disk may pass in about 500 microseconds while a sector near the outer diameter may pass in only about 300 microseconds. Consequently, if the target is the first sector of band N and the clock frequency changes after detection of the last sector mark of the last track of band N-1, target sector detection will be unreliable and may, in fact, be impossible. On the other hand, if the frequency is changed from f.sub.N-1 to f.sub.N while the head is still in band N-1, the remaining sector marks and ID information in band N-1 cannot be reliably read and windowing of sector marks cannot be used because controller synchronization can be lost. Other combinations of target sector locations and locations of sectors on which the laser beam lands relative to band boundaries also result in reduced reliability with which sector marks can be detected and data written or read. Consequently, it has been preferred to remap, or otherwise designate as unusable or spare, certain sectors or even tracks at or near band boundaries thereby wasting valuable disk capacity and increasing access time.